In electrophotography, a latent image is created on the surface of an imaging member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to the use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains electron transport material rather than a hole transport material, the charge placed on the photoconductor surface will be opposite that described herein.
When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer is comprised of the charge generation compound or molecule alone and/or in combination with a binder. The charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al. U.S. Pat. No. 5,130,215 and the Balthis et al. U.S. Pat. No. 5,545,499.
A common phenomena observed with dual layer organic photoconductors is positive electrical fatigue which causes lower residual potential with cycling. Photoconductor electrical fatigue is observed as a change in discharge voltage versus exposure energy upon electrical or print cycling. Positive photoconductor fatigue contributes to darkening print copy over life of a photoconductor.
Photoconductor drums are frequently handled by operators during drum inspection or cartridge assembly. Contamination of the photoconductor drum can occur by hand or food oils during this handling by operators. This contamination often leads to crazing of the photoconductor drum. Crazing is a term used to define the cracking of a polymer surface induced by contamination by hand or food oils. Crazing can effect the life and photoelectric qualities of the photoconductor.
As such, there is a need for photoconductors, charge generation layers and charge transport layers which increase photoconductor stability, reduce positive electrical fatigue, induce negative electrical fatigue in which the residual potential increases with cycling, and/or prevent or mitigate crazing of the drums.